Heat-sensitive recording materials using an electron donating colorless dye (color former) and an electron accepting compound (color developer) have been disclosed in Japanese Patent Publications No. 14039/70 (corresponding to U.S. Pat. No. 3,539,375) and No. 4160/68 (corresponding to U.S. patent application Ser. No. 512,546). Properties which are essentially required for the heat-sensitive recording materials are that (1) color density and color sensitivity must be sufficient, (2) fogging (a color formation phenomenon during preservation before using) is not caused, and (3) a colored product after color formation has sufficient fastness. However, heat-sensitive recording materials which completely satisfy these requirements have not been obtained up to now.
Particularly, studies relating to the above described requirement (1) have recently been earnestly carried out on means for speeding up of the heat-sensitive recording system.
One of the methods is to elevate the melting point of the electron accepting compound from 60.degree. C. to 100.degree. C. However, in phenol compounds which are most widely used at present as electron accepting compounds, it is difficult to control the melting point. Further, since the phenolic compounds are expensive, their practically is inferior.
Another method described in Japanese Patent Publications Nos. 17748/74 and 39567/76 uses an organic acid and a phenolic compound together as the electron accepting substances or a polyvalent metal salt of compounds having an alcoholic hydroxyl group. Further, it has been described in Japanese Patent Publication No. 29945/76 to use copolymers of hydroxyethyl cellulose and maleic acid anhydride salt.
Further, the addition of waxes has been described in Japanese Patent Publication No. 27599/76 and Japanese Patent Application (OPI) No. 19231/73 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application". ).
Additional approaches have been described in Japanese Patent Applications (OPI) Nos. 34842/74, 115554/74, 149353/75, 106746/77, 5636/78, 11036/78, 48751/78 and 72996/81. These approaches involve adding nitrogen containing organic compounds such as thioacetanilide, phthalonitrile, acetamide, di-.beta.-naphthyl-p-phenylenediamine, aliphatic acid amide, acetoacetanilide, diphenylamine, benzamide or carbazole, etc., thermofusible substances such as 2,3-di-m-tolylbutane or 4,4'-dimethylbiphenyl, etc., or carboxylic acid esters such as dimethyl isophthalate, diphenyl phthalate or dimethyl terephthalate, etc., as a sensitizer. Furthermore, it has been described in British Patent Publication No. 2,074,335A to add hindered phenols.
However, heat-sensitive recording materials produced by using these methods are insufficient with respect to color density and color sensitivity.